Optical recording disks and other optical recording media have attracted great attention as high capacity information recording media. The optical recording media include the rewritable type with phase change and magnetooptical properties, and the write-once type with pitforming.
The phase change optical recording media use recording film of a phase changeable alloy which shows its reflectance change between the crystalline and amorphous states or between two different crystalline states. For the phase change type optical recording media with different crystalline states, Ag-Zn alloy is a typical phase changeable alloy as described in Japanese Patent Application Kokai (JP-A) No. 130089/1986. This phase change type optical recording media having Ag-Zn alloy as the recording film, however, show a relatively small change in reflectivity due to the phase change, which cannot satisfy the reflectivity change specification required by the CD standard. The CD standard specifications must also require that information carrying areas, that is, recorded pits, have lower reflectivity than the other non recorded parts. The Ag-Zn alloy as a recording film has a general tendency that light exposed areas, that is, recorded areas increase its reflectivity. Therefore, Ag-Zn alloy cannot satisfy the CD standard by this reason. That is, these phase change type optical recording media with Ag-Zn alloys for recording cannot be used for write-once compact disks.
JP-A 236789/1990 proposes a new optical recording disk satisfying the CD standard. Disclosed is an optical information recording disk having a high reflectivity layer and a low reflectivity layer stacked on an upper surface of a substrate in this order. The high reflectivity layer is formed with an element selected from the group consisting of Au, Al, Ag, Pt, Pd, Ni, Cr, and Co, or an alloy containing such an element or elements, which has high reflectivity more than 70% for incident laser power so that it cannot be a recording material as such. The low reflectivity layer is formed with a material which has high absorption over the incident laser wavelength in the range of 750 to 850 nm, for example, a chalcogen element such as Te. Information is recorded in this optical recording disk by introducing recording light from the substrate upper surface side, that is, the low reflectivity layer side, whereby the chalcogen element of the low reflectivity layer reacts with the element or elements of the high reflectivity layer to form a new alloy. In this way, the light exposed areas reduce its optical reflectivity. Such a change in optical reflectivity can be detected by directing reproducing light with much smaller power than that of recording to the disk from the opposite side, that is, the substrate lower surface side. This configuration is described as providing write-once compact disks.
According to JP-A 235789/1990, the high and low reflectivity layers are formed by sputtering. We prepared optical recording disks of the disclosed configuration using a sputtering process and carried out recording and reproduction on the disks, finding that unrecorded areas had a reflectivity as low as about 14 to 16% and recorded areas had the reflectivity decreased to only about 10%. As a result, these disks could not be reproduced not only in the standard CD mode, but also by means of a driving equipment adapted for phase change type optical recording disks. When a low reflectivity layer of Te was formed on a high reflectivity layer of Ag by sputtering, the mutual diffusion occurred between the layers to form an alloy or a compound between Ag and Te, suggesting that a recorded state was established immediately during sputtering.
It is to be noted that these results were obtained when the high reflectivity layer was formed with about 500 A thick enough to carry out recording at a linear velocity of 1.2 to 1.4 m/s corresponding to the CD standard. By increasing the thickness of the high reflectivity layer up to about 1000 A, the influence of interdiffusion during the formation of the low reflectivity layer was reduced so that sufficient reflection is provided by the high reflectivity layer in unrecorded state. However, in this case, it took a long time for the layers to diffuse into each other and recording could not be completed by irradiating recording laser light at the linear velocity prescribed in the CD standard.
In Example 5 of JP-A 235789/1990, a Sb layer and a Te layer are stacked on a high reflectivity layer of Au in this order as low reflectivity layers. A recorded state can already be established at the time of forming the Sb layer since interdiffusion readily happens between Sb and Au.
A further problem arises with this optical information recording disk when recording and reproducing light beams are directed to the disk from the substrate front and the rear surface sides, respectively. This recording must require that the disk must be turned over also need a reversely rotational equipment. Thus a special drive is necessary for recording. Recording light is directed to the disk from the low reflectivity layer side because the high reflectivity layer has a high melting point and low absorption of incident light coefficient, therefore extremely high recording power is necessary if recording light is directed from the substrate sides without the layers.
Additionally, in the design of optical recording medium having such two thin films wherein interdiffusion causes change in its optical reflectivity, actual experiments must be necessary to confirm interdiffusion, thicknesses and other factors of the two films. Therefore, some methods which predict these properties mentioned above without actual experiments to save the time and labor cost are necessary.